A variety of inorganic substances are recently studied as carriers of drug in drug delivery systems. As for such carriers, nanoparticles are particularly noted, and hydroxyapatite crystals and nanoparticles on the basis of silica and the like have been reported so far.
Under these circumstances, there has been a growing interest in a functional nanoparticle, i.e. carbon nanoparticles such as a fullerene, carbon nanotubes, and carbon nanohorns.
Among these particles, carbon nanohorns (CNH) are recently discovered as a new type of carbon nanoparticles during the study of carbon nanotube preparation by the present inventors (see Non-patent Document 1). It has been found by using transmission electron microscopy (TEM), that the particles are nanostructured spherical aggregates of graphitic tubes of which the diameter is about 80 nm and most are attached with conical caps (horns). Each horn consists of completely closed single-wall graphitic sheet and its diameter is 2 to 3 nm, which is much larger than the 1.4 nm of typical single-wall carbon nanotubes.
In addition, an oxidation treatment produced nanowindows on the top or in the side wall of the horn, and it has been confirmed that various molecules can infiltrate into the inner space of the horn through the hole (see Non-patent Document 2). This enables to use the inner wall and internal cavity of the horn as a capture site of a substance, and greatly enlarges the surface area of the oxidized carbon nanohorns compared to non-oxidized carbon nanohorns. In fact, nitrogen gas can be not only adsorbed onto the interstices of the individual oxidized sigle-walled carbon nanohorns (SWNHox), but also included in the internal cavities of the horns. Diameters of the holes of side wall and top of the horn are measured as 1.58 nm and 1.17 nm, respectively on TEM photographs of SWNHox. The oxidation also introduces oxygen functional groups such as carboxylic groups and quinine groups at the pore edges of the SWNHox (see Non-patent Documents 3 and 4).
On the other hand, the present inventors also reported that the incorporation of fullerene (C60) into oxidized single-wall carbon nanotubes in liquid phase through the pores of tips and sidewalls (see Non-patent Document 5).
[Non-patent Document 1]
Iijima, S. et al., Chem. Phys. Lett. (1999) Vol. 309, pp. 165-170
[Non-patent Document 2]
Bekyarova, E. et al., Langmuir (2002) Vol. 18, pp. 4138-4141
[Non-patent Document 3]
Bekyarova, E. et al., J. Phys. Chem. (2003) Vol. 107, pp. 4479-4484
[Non-patent Document 4]
Kuznetsova, A. et al., Chem. Phys. Lett. (2000) Vol. 321, pp. 292-296
[Non-patent Document 5]
Yudasaka M. et al., Chem. Phys. Lett. (2003) Vol. 380, pp. 42-46